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Subject[tip:sched/core] sched/core: Remove get_cpu() from sched_fork()
Commit-ID:  af0fffd9300b97d8875aa745bc78e2f6fdb3c1f0
Gitweb: https://git.kernel.org/tip/af0fffd9300b97d8875aa745bc78e2f6fdb3c1f0
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate: Fri, 6 Jul 2018 15:06:15 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 16 Jul 2018 00:16:29 +0200

sched/core: Remove get_cpu() from sched_fork()

get_cpu() disables preemption for the entire sched_fork() function.
This get_cpu() was introduced in commit:

dd41f596cda0 ("sched: cfs core code")

... which also invoked sched_balance_self() and this function
required preemption do be off.

Today, sched_balance_self() seems to be moved to ->task_fork callback
which is invoked while the ->pi_lock is held.

set_load_weight() could invoke reweight_task() which then via $callchain
might end up in smp_processor_id() but since `update_load' is false
this won't happen.

I didn't find any this_cpu*() or similar usage during the initialisation
of the task_struct.

The `cpu' value (from get_cpu()) is only used later in __set_task_cpu()
while the ->pi_lock lock is held.

Based on this it is possible to remove get_cpu() and use
smp_processor_id() for the `cpu' variable without breaking anything.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180706130615.g2ex2kmfu5kcvlq6@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/core.c | 13 ++++---------
1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ba6bb805693a..c3cf7d992159 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2294,7 +2294,6 @@ static inline void init_schedstats(void) {}
int sched_fork(unsigned long clone_flags, struct task_struct *p)
{
unsigned long flags;
- int cpu = get_cpu();

__sched_fork(clone_flags, p);
/*
@@ -2330,14 +2329,12 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
p->sched_reset_on_fork = 0;
}

- if (dl_prio(p->prio)) {
- put_cpu();
+ if (dl_prio(p->prio))
return -EAGAIN;
- } else if (rt_prio(p->prio)) {
+ else if (rt_prio(p->prio))
p->sched_class = &rt_sched_class;
- } else {
+ else
p->sched_class = &fair_sched_class;
- }

init_entity_runnable_average(&p->se);

@@ -2353,7 +2350,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
* We're setting the CPU for the first time, we don't migrate,
* so use __set_task_cpu().
*/
- __set_task_cpu(p, cpu);
+ __set_task_cpu(p, smp_processor_id());
if (p->sched_class->task_fork)
p->sched_class->task_fork(p);
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
@@ -2370,8 +2367,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
plist_node_init(&p->pushable_tasks, MAX_PRIO);
RB_CLEAR_NODE(&p->pushable_dl_tasks);
#endif
-
- put_cpu();
return 0;
}

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